Received Orthogonal Frequency Division Multiplexing Symbols Research Articles

A New OFDM System for IIR Channels

In this paper, we propose a new orthogonal frequency division multiplexing (OFDM) system for an infinite impulse response (IIR) channel with the form of B(z)/A(z) for two polynomials A(z) and B(z). Different from the conventional OFDM transmission over a finite impulse response (FIR) channel, a guard interval of an OFDM symbol is added such that the corresponding part at receiver is the cyclic prefix (CP) of the received OFDM symbol. The guard interval and CP lengths are the same and not smaller than the orders of polynomials A(z) and B(z). The OFDM symbol without the guard interval is the same as the conventional OFDM symbol without the CP. At the receiver, the IIR channel is then converted to N intersymbol interference (ISI) free subchannels, where N is the number of subcarriers of an OFDM symbol.

Multi-output Convolutional Neural Network Based Distance and Velocity Estimation Technique for Orthogonal Frequency Division Multiplexing Radar Systems

The objective of this work is to propose a new method of estimating velocity and distance based on multi-output convolutional neural network (CNN) for orthogonal frequency division multiplexing (OFDM) radars. The two-dimensional (2D) periodogram is extracted from the received reflected waveforms through radar signal processing of received OFDM symbols. Conventionally, constant false alarm rate (CFAR) algorithm is used to estimate distance and velocity of targets. In contrast, this paper proposes a novel deep-learning based approach for the estimation of the targets in OFDM radar systems. The proposed multi-output CNN-based target detector estimates the distance and velocity of the target simultaneously. The proposed technique is verified through computer simulation. The results show that the proposed multi-output CNN-based method demonstrates more accurate distance and speed estimates than the conventional CFAR. Specifically, the distance and speed estimates of the proposed method are 9.8 and 12.3 times accurate, respectively, than those of the conventional CFAR.

Multiple Target Detection For OFDM Radar Based On Convolutional Neural Network

The objective of this paper is to propose a multiple target identification technique for orthogonal frequency division multiplexing (OFDM) radars. First, a 2-D (range & Doppler) periodogram is obtained from the reflected signal through 2-D fast Fourier transform (FFT) of the received OFDM symbols. Usually, the peaks of the periodogram indicates the targets. Conventionally, peak search algorithms are used to find the multiple targets. In this paper, however, a convolutional neural network (CNN) classifier is proposed to identify the targets. The proposed technique does not need any additional information but the 2-D periodogram while the conventional method requires the noise variance as well as the periodogram. The performance is examined through computer simulation. According to the results, if the number of maximum identifiable targets are small, the proposed technique performs well. However, as the number increases, the detection accuracy decreases. In the simulation environments, the proposed method outperforms the conventional one. The proposed OFDM radar technique can be applied to 6G mobile communications to identify the moving targets around the transmitter without additional frequency resource for radar systems.

Adaptive Filtering Techniques using Cyclic Prefix in OFDM Systems for Multipath Fading Channel Prediction

This paper presents adaptive channel prediction techniques for wireless orthogonal frequency division multiplexing (OFDM) systems using cyclic prefix (CP). The CP not only combats intersymbol interference, but also precludes requirement of additional training symbols. The proposed adaptive algorithms exploit the channel state information contained in CP of received OFDM symbol, under the time-invariant and time-variant wireless multipath Rayleigh fading channels. For channel prediction, the convergence and tracking characteristics of conventional recursive least squares (RLS) algorithm, numeric variable forgetting factor RLS (NVFF-RLS) algorithm, Kalman filtering (KF) algorithm and reduced Kalman least mean squares (RK-LMS) algorithm are compared. The simulation results are presented to demonstrate that KF algorithm is the best available technique as compared to RK-LMS, RLS and NVFF-RLS algorithms by providing low mean square channel prediction error. But RK-LMS and NVFF-RLS algorithms exhibit lower computational complexity than KF algorithm. Under typical conditions, the tracking performance of RK-LMS is comparable to RLS algorithm. However, RK-LMS algorithm fails to perform well in convergence mode. For time-variant multipath fading channel prediction, the presented NVFF-RLS algorithm supersedes RLS algorithm in the channel tracking mode under moderately high fade rate conditions. However, under appropriate parameter setting in $$2\times 1$$2?1 space---time block-coded OFDM system, NVFF-RLS algorithm bestows enhanced channel tracking performance than RLS algorithm under static as well as dynamic environment, which leads to significant reduction in symbol error rate.

Blind and semi-blind channel estimation with fast convergence for MIMO-OFDM systems

In this paper, the blind subspace channel estimation using the block matrix scheme is proposed for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. Based on the Toeplitz structure, the block matrix scheme collects a group of the received OFDM symbols into a vector, and then partitions it into a set of equivalent symbols. The number of equivalent symbols is about N times of OFDM symbols, where N is the size of FFT operation. With those equivalent symbols, the proposed blind subspace channel estimation can converge within a small amount of OFDM symbols. The identifiability of the proposed channel estimation is examined that the channel matrix is determined up to an ambiguity matrix. Besides, the semi-blind channel estimation is also investigated by combining few pilot sequences with the subspace method. Simulation results show that the proposed channel estimators perform very well even in a time-varying scenario. Especially the semi-blind methods achieve almost the same BERs as those by true channels.

Performance enhancement of wireless mobile adhoc networks through improved error correction and ICI cancellation

Mobile adhoc network (MANET) refers to an arrangement of wireless mobile nodes that have the tendency of dynamically and freely self-organizing into temporary and arbitrary network topologies. Orthogonal frequency division multiplexing (OFDM) is the foremost choice for MANET system designers at the Physical Layer due to its inherent property of high data rate transmission that corresponds to its lofty spectrum efficiency. The downside of OFDM includes its sensitivity to synchronization errors (frequency offsets and symbol time). Most of the present day techniques employing OFDM for data transmission support mobility as one of the primary features. This mobility causes small frequency offsets due to the production of Doppler frequencies. It results in intercarrier interference (ICI) which degrades the signal quality due to a crosstalk between the subcarriers of OFDM symbol. An efficient frequency-domain block-type pilot-assisted ICI mitigation scheme is proposed in this article which nullifies the effect of channel frequency offsets from the received OFDM symbols. Second problem addressed in this article is the noise effect induced by different sources into the received symbol increasing its bit error rate and making it unsuitable for many applications. Forward-error-correcting turbo codes have been employed into the proposed model which adds redundant bits into the system which are later used for error detection and correction purpose. At the receiver end, maximum a posteriori (MAP) decoding algorithm is implemented using two component MAP decoders. These decoders tend to exchange interleaved extrinsic soft information among each other in the form of log likelihood ratio improving the previous estimate regarding the decoded bit in each iteration.

Blind Compensation for Phase Noise in OFDM Systems over Constant Modulus Modulation

A blind algorithm is proposed in order to compensate for the phase noise resulting from imperfect oscillators in orthogonal frequency-division multiplexing (OFDM) systems over constant modulus modulation. In the proposed algorithm, one received OFDM symbol is partitioned into subblocks in the time domain and the phase noise over each subblock is approximated as its time-average. Under the approximation, the squared magnitude of the channel gain multiplied by the data symbol at each subcarrier is shown to be expressed in terms of these time-averages and the discrete Fourier transform (DFT) coefficients of the received samples at each subblock with zero padding. Based on the relation, the proposed algorithm compensates for the phase noise without pilot symbols. Numerical results show that the proposed algorithm outperforms conventional algorithms as well as it requires lower computational complexity.

Space-alternating generalised expectation–maximisation-based H-infinity channel estimator for multiple-input multiple-output-orthogonal frequency division multiplexing systems

This study presents a low-complexity and robust H-infinity channel estimator for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. The H-infinity estimator, which has never been devoted to MIMO-OFDM systems, could be implemented by applying a multiple-order auto-regression (AR) model. However, it may increase the design complexity of receivers and lead to poor real-time property when this model is used for MIMO-OFDM systems, which makes the authors abandon AR model. In order to reduce the number of matrices manipulations because of the each received OFDM symbols from different transmit antennas, the iterative space-alternating generalised expectation–maximisation (SAGE) algorithm is adopted. Furthermore, to deal with the effect of non-Gaussian noise (NGN) channels, because of various natural or man-made impulsive sources, an equivalent signal model (ESM) is introduced to alleviate the effect of this issue and enhance the robust of SAGE-based H-infinity estimator. Simulation results show that H-infinity estimator has almost the same bit error rate performance as optimal maximum a posteriori estimator. The performance gain afforded by using ESM can be substantial when compared with using the traditional signal model, which dramatically enhance the robustness of SAGE-based H-infinity estimator against NGN channels.

OFDM에서 블라인드 주파수 옵셋 추정 방법

In this paper, an efficient blind carrier frequency offset (CFO) estimation method in orthogonal frequency division multiplexing (OFDM) systems is proposed. In the proposed method, we obtain two time different received OFDM symbols by using both the cyclic prefix and oversampling technique, and a cost function is defined by using the two OFDM symbols. We show that the cost function can be approximately expressed as a cosine function. Using a property of the cosine function, a formular for estimating the CFO is derived. The estimator of the CFO requires three independent cost function values calculated at three different points of frequency offset. The proposed method is very efficient in computational complexity since no searching operation for the minimum cost value is required. The proposed method reduces 97% of the amount of FFT computation, compared with the ML method. Unlike the conventional methods such as the ML method and the MUSIC] method, the accuracy of the proposed method is independent of the searching resolution since the closed form solution exists. The computer simulation shows that the performance of the proposed method is superior to those of the MUSIC and the ML method.

Iterative Receivers Based on Subblock Processing for Phase Noise Compensation in OFDM Systems

An iterative algorithm employing decision feedback provided by either an equalizer or a channel decoder is proposed in order to compensate for the phase noise resulting from imperfect oscillators in orthogonal frequency-division multiplexing (OFDM) systems. In the proposed algorithm, the received OFDM symbol is partitioned into subblocks in the time domain and the estimate of the time-average of the phase noise at each subblock is used to compensate for the phase noise. A formula for the signal-to-interference-plus-noise ratio (SINR) after phase noise compensation is derived and its values are evaluated under a variety of conditions in order to demonstrate the efficiency of the proposed algorithm. Numerical results show that the proposed algorithm achieves performance close to that of an OFDM system without phase noise over a wide range of conditions, while requiring only one-half the complexity of the partial intercarrier interference reduction algorithm.

A fine carrier recovery algorithm robustto doppler shift for OFDM systems

In this paper, a fine carrier recovery algorithm is proposed to maintain the synchronization even in a severe Doppler shift for orthogonal frequency division multiplexing (OFDM) systems such as digital video broadcasting for handheld (DVB-H), DVB for terrestrial (DVB-T) and DVB-T2. The proposed algorithm estimates the frequency offset by utilizing correlation values in the received OFDM symbols by adding intentional frequency offsets. Its performance is compared with conventional algorithms for the DVB-H system in fading with Doppler.

An EM-Based Semiblind Joint Channel and Frequency Offset Estimator for OFDM Systems Over Frequency-Selective Fading Channels

We propose an estimator that will jointly estimate the channel and the carrier frequency offset (CFO) for orthogonal frequency-division multiplexing (OFDM) systems based on the maximum-likelihood criterion. The proposed estimator uses only one received OFDM symbol to perform estimation and does not require the knowledge of power delay profile, except that the number of delay paths should be known. The search range for the CFO is first partitioned into a number of subranges. The expectation-maximization (EM) and Newton's methods are then used within each subrange in an alternative manner to jointly obtain the channel and the CFO estimates. By assuming that the transmitted time-domain OFDM signals can be modeled as Gaussian random variables, we show that the EM method has closed-form expressions and, thus, can be efficiently implemented. We then study the mean-square-error (MSE) performance of the estimator under various signal-to-noise (SNRs) ratios and various numbers of cyclic prefix samples through simulations. Finally, we compare the performance of our proposed estimator with that of some existing estimators in terms of bit error rate (BER). Our simulation results show that the proposed estimator performs better than the reported estimator, which sequentially estimates the channel and the CFO. By suitably choosing the number of subranges, our estimator also performs better than a reported estimator, which makes use of pilot symbols to obtain the initial CFO and channel estimates and then uses a decision-directed technique for overall eventual estimation.

Semi-Blind Time-Domain Equalization for MIMO-OFDM Systems

In this paper, a semi-blind time-domain equalization technique is proposed for general multiple-input-multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems. The received OFDM symbols are shifted by <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">more</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">than</i> or <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">equal</i> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">to</i> the cyclic prefix (CP) length, and a blind equalizer is designed to completely suppress both intercarrier interference (ICI) and intersymbol interference (ISI) using second-order statistics of the shifted received OFDM symbols. Only a one-tap equalizer is needed to detect the time-domain signals from the blind equalizer output, and one pilot OFDM symbol is utilized to estimate the required channel state information for the design of the one-tap equalizer. The technique is applicable irrespective of whether the CP length is longer than, equal to, or shorter than the channel length. Computer simulations show that the proposed technique outperforms the existing techniques, and it is robust against the number of shifts in excess of the CP length.

Double Window Cancellation and Combining for OFDM in Time-Invariant Large Delay Spread Channel

In a time-invariant wireless channel, the multipath that exceeds the cyclic prefix (CP) or the guard interval (GI) causes orthogonal frequency division multiplexing (OFDM) systems to hardly achieve high data rate transmission due to the inter-symbol interference (ISI) and the inter-carrier interference (ICI). In this paper the new canceller scheme, named as Double Window Cancellation and Combining (DWCC) is proposed. It includes the entire symbol interval, delayed by multipath as a signal processing window and intends to improve the performance by combining the double windows that can be formed by the pre-and post-ISI cancellation and reconstruction to the received OFDM symbol interfered by the multipath exceeding the guard interval. The proposed scheme has two algorithm structures of the DWCC-I and -II which are distinguished by the operational sequence (Symbol-wise or Group-wise) to the OFDM symbols of the received packet and by the selection of the processing window in the iterative decision feedback processing. Since the performance of the canceller is dependant on the equalization, particularly on the initial equalization, the proposed schemes operate with the time and frequency domain equalizer in the initial and the iterative symbol detection, respectively. For the verification of the proposed schemes, each scheme is evaluated in the turbo coded OFDM for low (QPSK) and high level modulation systems (16QAM, 64QAM), and compared with the conventional canceller with respect to the performance and computational complexity. As a result, the proposed schemes do not have an error floor even for 64QAM in a severe frequency selective channel.